The present invention relates to a nonaqueous electrolyte secondary battery, and more particularly to a nonaqueous electrolyte secondary battery capable of highly efficient charge and discharge.
A typical one of the nonaqueous electrolyte secondary batteries is a lithium ion secondary battery having a negative electrode which may be doped and de-doped with lithium and a positive electrode including a transition metal oxide. The negative electrode comprises a stripe-shaped negative electrode collector applied with a negative electrode active material. The positive electrode comprises a stripe-shaped positive electrode collector applied with a positive electrode active material. The negative electrode and the positive electrode sandwich a separator to form a lamination structure. The lamination structure is enclosed in a package. Alternatively, it is possible that the lamination structure is rolled to form a cylindrically shaped battery element so called as a Jelly-roll so that the cylindrically shaped battery element is accommodated in a cylindrically shaped battery can.
The cylindrically shaped battery is superior in seal-ability and allows a uniform battery reaction over sites of the cylindrically shaped battery element. If the nonaqueous electrolyte secondary battery is required to supply a large current, then the cylindrically shaped battery is suitable and important. The cylindrically shaped nonaqueous electrolyte secondary battery is attractive as a large battery for electric car, electric auxiliary bicycle and the like.
FIG. 1A is a cross sectional elevation view illustrative of a conventional cylindrically shaped nonaqueous electrolyte secondary battery. FIG. 1B is a view illustrative of a pair of positive and negative electrodes included in the conventional cylindrically shaped nonaqueous electrolyte secondary battery shown in FIG. 1A. FIG. 1C is a schematic perspective view illustrative of a cylindrically shaped battery element comprising a rolled structure of laminations of a separator sandwiched by positive and negative electrodes included in the conventional cylindrically shaped nonaqueous electrolyte secondary battery shown in FIG. 1A.
A cylindrically shaped nonaqueous electrolyte secondary battery 51 comprises a cylindrically shaped battery can 52 and a cylindrically shaped battery element 56 contained in the cylindrically shaped battery can 52. The cylindrically shaped battery element 56 comprises a rolled structure of laminations of stripe-shaped positive and negative electrodes 54 and 53 sandwiching a stripe-shaped separator 55. The stripe-shaped positive electrode 54 comprises a stripe-shaped positive electrode collector applied with a positive electrode active material. The stripe-shaped negative electrode 53 comprises a stripe-shaped negative electrode collector applied with a negative electrode active material. The stripe-shaped separator 55 is wider in width than the stripe-shaped negative and positive electrodes 53 and 54, so that opposite sides of the laminations comprise opposite sides of the stripe-shaped separator 55. Thus, opposite ends of the rolled structure of the laminations comprise opposite sides of the stripe-shaped separator 55. The cylindrically shaped battery can 52 may serve as a negative electrode side terminal, wherein the stripe-shaped negative electrode 53 is electrically connected to the cylindrically shaped battery can 52 through a negative electrode tab 57. A first end of the negative electrode tab 57 is bonded to the stripe-shaped negative electrode 53 and a second end of the negative electrode tab 57 is bonded by welding to an inner wall of the cylindrically shaped battery can 52.
If the cylindrically shaped nonaqueous electrolyte secondary battery is required to perform highly efficient charge and discharge operations, then a plurality of negative electrode tabs are bonded by welding to the inner wall of the cylindrically shaped battery can in order to reduce an IR-loss and allow a uniform battery reaction.
Positive electrode tabs 58 are also provided. A first end of each of the positive electrode tabs 58 is bonded to the stripe-shaped positive electrodes 54 and a second end of the positive electrode tabs 58 is bonded to a battery header 59 which serves as a positive electrode side terminal. The battery header 59 is provided with a pressure release valve or a pressure control valve for releasing an internal pressure of the battery if the internal pressure is excessively increased.
As shown in FIG. 1B, the stripe-shaped positive electrode 54 comprises the stripe-shaped positive electrode collector applied with the positive electrode active material except for a one-side active material free region. Namely, the one side active material free region is not applied with the positive electrode active material. The positive electrode tabs 58 are bonded by welding to the one-side active material free region of the stripe-shaped positive electrode collector of the stripe-shaped positive electrode 54. Also, the stripe-shaped negative electrode 53 comprises the stripe-shaped negative electrode collector applied with the negative electrode active material except for a one-side active material free region. Namely, the one side active material free region is not applied with the negative electrode active material. The negative electrode tabs 57 are bonded by welding to the one-side active material free region of the stripe-shaped negative electrode collector of the stripe-shaped negative electrode 53. The stripe-shaped negative and positive electrodes 53 and 54 sandwich the stripe-shaped separator 55 to form the lamination. The lamination is then rolled to form the rolled structure of the lamination, wherein the rolled structure forms the cylindrically shaped battery element as shown in FIG. 1C. The cylindrically shaped battery element is accommodated in the cylindrically shaped battery can 52. The negative electrode tabs 57 and the positive electrode tabs 58 are so long as to increase the losses of currents at the negative electrode tabs 57 and the positive electrode tabs 58. It is also difficult to make uniform the length of the plural electrode leads, the current losses are likely to be non-uniform between the negative electrode tabs 57 and the positive electrode tabs 58. This may cause that part of the negative electrode tabs 57 and the positive electrode tabs 58 shows a heat generation. The current distribution is different between the adjacent part and far apart from the bonding part of each of the negative electrode tabs 57 and the positive electrode tabs 58. Those make it difficult to realize the highly efficient charge and discharge operations.
In Japanese laid-open patent publication No. 7-6749, it is disclosed that a cylindrically shaped secondary battery has positive and negative electrodes and collector terminals with comb-teeth shaped welding portions. This publication also discloses a method of the cylindrically shaped secondary battery.
This conventional method utilizes a spot welding where a welding current is concentrated to projecting portions of the collector. The available electrode materials are limited to materials such as nickel allowing the spot welding. it is also necessary that the separator is thermally stable to the heat generation due to the spot welding.
In Japanese laid-open patent publication No. 9-306465, it is disclosed that a cylindrically shaped secondary battery increases in the number of connecting parts between the positive and negative electrodes and the collectors from the most inner portion to the most outer portion of the cylindrically shaped battery element.
The positive and negative electrodes are connected to the collectors by the spot welding. This spot welding is carried out by plural times of contact of a welding rod to different positions, for which reason it is difficult to realize the exactly desired welding. Further, the electrode material is limited to the material allowing the spot welding.
In the above circumstances, it had been required to develop a novel nonaqueous electrolyte secondary battery free from the above problem.
Accordingly, it is an object of the present invention to provide a novel nonaqueous electrolyte secondary battery free from the above problems.
It is a further object of the present invention to provide a novel nonaqueous electrolyte secondary battery with a reduced IR-loss by electrode tabs between a battery element and a battery can or a battery cap in highly efficient charge and discharge operations.
It is a still further object of the present invention to provide a novel nonaqueous electrolyte secondary battery showing a uniform current distribution over positions of a battery element.
The present invention provides a nonaqueous electrolyte secondary battery having a cylindrically shaped battery element comprising positive and negative electrodes and a separator sandwiched between the positive and negative electrodes. Each of the positive and negative electrodes has an active material free region, on which no active material is applied. The active material free region has a projecting edge region which projects or extends beyond a first side edge of the separator, wherein a first end of the cylindrically shaped battery element comprises the projecting edge region of the active material free region of one of the positive and negative electrodes, and the first end of the cylindrically shaped battery element has a depressed portion. An electrode tab has a first portion engaged within and welded to the depressed portion and a second portion electrically in contact with an electrode terminal.
The above and other objects, features and advantages of the present invention will be apparent from the following descriptions,